Such a mass flowmeter is known from U.S. Pat. No. 4,658,657.
The known mass flowmeter comprises a loop-shaped tube (half a turn) that forms a transverse branch at one side and two lateral branches clamped in at the opposite side in a mounting beam. The latter is mounted in a support such that it can rotate about a central axis lying in the plane of the loop. An electromagnetic excitation system cooperating with the mounting beam provides an oscillatory rotation (rotational vibration) of the mounting beam with the loop about the central axis. (The term ‘excitation’ is here understood to mean ‘causing to oscillate’).
When a medium (gas or liquid) flows through the loop that rotates about the central axis, Coriolis forces are generated in the transverse branch, resulting in an oscillation of the loop about a secondary axis. This oscillation, which is proportional to the flow, is superimposed on the fundamental oscillation and leads to a phase shift between the oscillations performed by the ends of the transverse branch. The phase difference is proportional to the Coriolis force and accordingly to the flow.
It is a disadvantage of the known system, however, that the mounting beam used for the excitation of the loop constitutes an additional mass. This prevents a change in the excitation frequency as a function of the density of the medium flowing through the tube, with the result that a measurement of the density (an additional property of a Coriolis flowmeter) becomes less accurate.
The invention has for its object inter alia to provide a flowmeter with an excitation system that is capable of measuring the density more accurately.
The mass flowmeter of the kind mentioned in the opening paragraph is for this purpose characterized in that the excitation means are (electro)magnetic and in operation do not make contact with the tube and have no components that are fastened to the tube. In other words: the excitation means are free from the movable portion of the tube both in the idle and in the operational state. It is noted that the tube may be, for example, a straight tube, a looped tube forming half a turn, a looped tube forming a full turn, or a looped tube forming a double turn.
The mass flowmeter according to the invention has an enhanced sensitivity because the (movable portion of the) tube is free from additional excitation components (=no added mass). A possibility of causing the tube to rotate is found in the use of a tube of magnetizable metal material, such as soft iron, in combination with one or two electromagnetic coils that can be energized in a pulsating mode (i.e. utilizing the natural magnetism of the tube material: operating in the way of a relay).